1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and its fabrication method, and more particularly, to an LCD fabricated by a simplified fabrication method to improve the production yield and picture quality of the LCD by reducing the number of masks.
2. Description of the Related Art
Recently, as the users' interest in information displays is growing and the demand for portable (mobile) information devices is increasing, research and commercialization of light and thin flat panel displays (FPD) to replace the existing display device, i.e., CRT (Cathode Ray Tube), are actively ongoing.
Among FPDs, the LCD, a device for displaying images by using optical anisotropy of liquid crystal materials, provides excellent resolution, color, and picture quality. Therefore, the LCD is widely used for notebook computers or desktop monitors and the like.
The LCD includes a color filter substrate (a first substrate), an array substrate (a second substrate), and a liquid crystal layer formed between the color filter substrate and the array substrate.
Generally, thin film transistors (TFTs) are used as switching elements of the LCD, and an amorphous silicon thin film is used as a channel layer of the TFT.
The fabrication process of the LCD requires a plurality of masking processes (namely, photolithography processes) to fabricate the array substrate including the TFTs. Therefore, a method for reducing the number of masking processes is necessary in terms of productivity.
The structure of a general LCD will be described in detail with reference to FIG. 1.
FIG. 1 is an exploded perspective view showing a general LCD.
As shown in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes color filters (C) having a plurality of sub-color filters 7 implementing red, green and blue colors, a black matrix 6 for separating the sub-color filters 7 and blocking light transmission to the liquid crystal layer 30, and a transparent common electrode 8 for applying a voltage to the liquid crystal layer 30.
The array substrate 10 includes a plurality of gate lines 16 and a plurality of data lines 17 crossing each other to define a plurality of pixel regions (P), TFTs formed at each crossing of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on each pixel region (P).
The color filer substrate 5 and the array substrate 10 are attached to each other by a sealant (not shown) formed at an outer edge of an image display region, and the two substrates 5 and 10 are attached by an attachment key (not shown) formed on the color filter substrate 5 or the array substrate 10.
FIGS. 2A to 2E are sectional views sequentially showing a fabrication process of the array substrate of the LCD in FIG. 1.
As shown in FIG. 2A, a gate electrode 21 made of a conductive material is formed by using a photolithography process (a first masking process) on a substrate.
Next, as shown in 2B, a first insulation film 15A, an amorphous silicon thin film and an n+ amorphous silicon thin film are sequentially deposited on the entire surface of the substrate 10 with the gate electrode 21 formed thereon, and the amorphous silicon thin film and the n+ amorphous silicon thin film are selectively patterned by using another photolithography process (a second masking process) to form an active pattern 24 of the amorphous silicon thin film on the gate electrode 21.
Here, the n+ amorphous silicon thin film pattern 25 which has been patterned in the same form as the active pattern 24 is formed on the active pattern 24.
Thereafter, as shown in FIG. 2C, a conductive metal material is deposited on the entire surface of the substrate 10 and then selectively patterned by using another photolithography process (a third masking process) to form a source electrode 22 and a drain electrode 23 at an upper portion of the active pattern 24. At this time, a certain portion of the n+ amorphous silicon thin film pattern formed on the active pattern 24 is removed through the third masking process to form an ohmic contact layer 25′ between the active pattern 24 and the source and drain electrodes 22 and 23.
Subsequently, as shown in FIG. 2D, a second insulation film 15B is deposited on the entire surface of the substrate 10 with the source electrode 22 and the drain electrode 23 formed thereon, and a portion of the second insulation film 15B is removed through another photolithography process (a fourth masking process) to form a contact hole 40 exposing a portion of the drain electrode 23.
Finally, as shown in FIG. 2E, a transparent conductive metal material is deposited on the entire surface of the substrate 10 and then selectively patterned by using another photolithography process (a fifth making process) to form a pixel electrode 18 electrically connected to the drain electrode 23 via the contact hole 40.
As mentioned above, it requires five (5) photolithography processes to fabricate the array substrate including the TFTs to pattern the gate electrode, the active pattern, the source and drain electrodes, the contact hole and the pixel electrode.
The photolithography process is a process of transferring a pattern formed on a mask onto the substrate on which a thin film is deposited to form a desired pattern, each of which includes a plurality of processes such as a process of coating a photosensitive solution, an exposing process and a developing process, etc. As a result, multiple photolithography processes may degrade the production yield and increase the probability of generating a defective TFT.
In particular, because the masks designed for forming the pattern is quite expensive, as the number of masks applied for the processes increases, the fabrication cost of the LCD increases proportionally.